Gas carburization and carbonitriding

ABSTRACT

The formation of soot is avoided during gas carburization or carbonitriding of steel having a carbon content of from 0.10 0.40 percent in an atmosphere of nitrogen and a hydrocarbon gas, such as natrual gas by the use of a barium, calcium or strontium containing inhibitor, for example, barium hydroxide.

United States Patent Hoffman et a1. [45] May 16, 1972 GAS CARBURIZATION AND 246,861 9/1881 CARBONITRIDING 1,848,958 3/1932 1,958,575 5/1934 [72] Inventors: Maurice F. Hoffman, Whippany; Abram 2 0 9 5 5 937 L. Hodge, Cranford, both of N .J [73] Assignee: Union Carbide Corporation, New York, FOREIGN PATENTS OR APPLICATIONS N.Y. 573,085 11/1945 Great Britain ..148/l6.5 [22] Filed: 1969 Primary Examiner-Charles N. Lovell 2 App[ 3 305 AtzorneyPaul A. Rose, Thomas 1. O'Brien, Harrie M.

Humphries and Lawrence G. Kastriner [52] US. Cl ..148/16.5, 148/166 57 ST CT [51] Int. Cl 1 ..C23c ll/l2,C23c11/18 58 Field of Search ..14s/12.1, 14, 15.5, 16.5, 16.6 The fmmatw" of avolded. durmg gas carburlzatlon or carbonitriding of steel having a carbon content of from 0.10 56 1 References Cited 0.40 percent in an atmosphere of nitrogen and a hydrocarbon gas, such as natrual gas by the use of a barium, calcium or UNITED STATES PATENTS strontium containing inhibitor, for example, barium hydroxide. 1,932,032 10/1933 Cowan ..148/16.5 2,240,146 4/1941 Ness 148/165 10 Claims, N0 Drawings GAS CARBURIZATION AND CARBONITRIDING BACKGROUND The present invention relates to a method for case hardening steel, and more particularly to a method for carburizing and carbonitriding of steel in the presence of a carbon containing gas mixture.

In order to improve the surface characteristics of steel, a variety of finishing processes are conventionally used. Case hardening is a method whereby carbon is diffused at high temperature into the surface layer of steel and caused to react with the iron. Carburization causes only carbon to be diffused into the iron, whereas carbonitriding is a case hardening process in which steel is held at an elevated temperature in a gaseous atmosphere from which it absorbs both carbon and nitrogen. The function of a carburizing atmosphere is to supply a sufficient quantity of carbon for absorption and diffusion into the steel. The main sources of such carbon are carbonaceous gases and easily vaporized hydrocarbon liquids. If carbonitriding is to take place, ammonia or an other gas which provides nascent nitrogen is included in the atmosphere as the source of nitrogen. Diffusion of carbon into the surface of the steel and reaction with the iron are generally carried out at a temperature in the range of from about 1,450 to 1,750 F preferably in the range of l,600 to l,700 F. To obtain a fully hardened case, the steel is generally quenched into a quenching medium such as water or oil. A variety of endothermically or exothermically generated carbon containing gases are presently used as the source of carbon for case hardening. Frequently, the production of these carbon containing gases, such as CO, require elaborate equipment in order to obtain the partial decomposition of the carbon containing gases which are then used for the carburizing atmosphere.

A carburizing gas consisting of nitrogen, hydrogen and natural gas would render case hardening more economical from the standpoint of initial capital investment and lower operating costs and would provide closer control of the carbon content than is possible with conventionally used case hardening atmospheres. However, attempts at case hardening with natural gas mixtures have in the past resulted in sooting and poor carburization. Sooting is believed to be caused by the fact that more carbon is yielded from the decomposition of the carburizing gas atmosphere than the iron in the steel is capable of absorbing. Consequently, the excess carbon formed by the gas is precipitated and deposited upon the articles in the carburizing furnace. Sooting is obviously undesirable because it results in unclean work, uneven case hardening and retardation of the carburizing or carbonitriding reactions.

OBJECTS It is an object of this invention to provide a method for case hardening (carburizing or carbonitriding) of steel in a gaseous atmosphere containing nitrogen and a hydrocarbon gas such as natural gas without sooting.

It is another object of this invention to provide a method for case hardening steel which is economical to operate and which requires a relatively low initial capital investment as compared with present commercial practice.

SUMMARY OF INVENTION These and other objects, which will become apparent from the detailed disclosure and claims to follow, are achieved by the present invention which provides a method for case hardening steel comprising: heating steel having a carbon content from about 0.10 0.40 percent to a temperature in the range of from about l,450 to 1,750 E, for a period of about 3 to 5 hours, in contact with a gaseous atmosphere comprising an inert carrier gas of nitrogen with or without added hydrogen, and a hydrocarbon gas selected from the group consisting of methane, ethane, propane, natural gas and mixtures thereof, in the presence of a barium, calcium or strontium containing inhibitor. The inhibitor may be used in the form of the pure metal, as an alloy of the metal or as a compound of the metal element.

DETAILED DESCRIPTION A variety of barium, calcium and strontium compounds have been found useful as inhibitors in accordance with this invention, particularly the oxides, carbonates, hydroxides, and halides of these metals. Suitable materials include BaO, BaCO Ba(OH) BaCl CaO, CaCO CaCl,, Ca(OH) SrO, SrCl SrCO and Sr(Ol-l),. However, the inhibitors need not be in the form of compounds; they may be used as powders of pure metal or alloys bearing these metals. Furthermore, it has been found that only very small amounts of the inhibitor need to be employed. Thus, for example, about 100 grams of inhibitor have been found sufficient to carburize 1,000 pounds of steel. It has also been found that the inhibitor in powdery or granular form may be dusted on the surface of the steel articles to be case hardened prior to being heated up to temperature in the furnace, or the carburizing gas atmosphere may be passed over a heated bed of the inhibitor having a temperature of at least 200 F. prior to coming in contact with the steel to be case hardened. It is also possible to construct the furnace lining of material containing the inhibitor.

The following examples will illustrate one method of carrying out the present invention. A charge of steel articles having a carbon content of from 0.10 0.40 percent to be case hardened including gears, shafts and bearing parts were introduced into a conventional carburizing furnace which was then brought up to carburizing temperature while the carburizing atmosphere was circulated around the charge. The specific compositions of the carburizing atmospheres used are shown in the table below (see Examples 1-3). During the carburization period, which may'last from 3 to 5 hours depending upon the depth of the case desired, the carbon from the decomposed hydrocarbon gas diffuses into the surface layer of the charge. In the case of carbonitriding, ammonia is also added to the atmosphere. The specific compositions of the atmospheres are shown in the table below (see Examples 4 and 5 In general, it has been found that the hydrocarbon gas content of the atmosphere should constitute about 2 to 12 percent (preferably 3 to 5 percent) of the total gas by volume with the remaining 88 to 98 percent consisting of a carrier gas. The carrier gas should contain to 100 percent nitrogen and 0 to 10 percent hydrogen. Only traces of inhibitor appear to be required in the gas phase to be effective in elimination of sooting. After the charge has been held at the desired temperature for a sufficient length of time to obtain the desired depth of case, the charge is quenched in a conventional oil bath. The results obtained from a series of five test heats are shown in the table below. Examples l3 were carburized for 3 hours at l,730 F., while Examples 4 and 5 were carbonitrided for 5 hours at l,450 F.

TABULATED RESULTS Satis- Atmosphero (vol. percent) factory uuso Exnmplo N- ll: N11. Nlli lnhihitor depth Sooting 10.5 llit(()ll)-g NOllt. 3.8 HMO): None. 4.6 HMO): Noun. 3.5 2.5 BMOII 2 None 2.9 2.5 1311(011): Ycs None *N.G.=Natural gas.

What is claimed is:

1. In a method for case hardening carbon steel having a carbon content of from about 0.10 0.40 percent comprising heating said carbon steel to a temperature of from about 1,450 to l,750 F. for from about 3 to 5 hours, the improvement comprising: elimination of sooting by carrying out the said heating step in a gaseous atmosphere comprising an inert carrier gas comprising nitrogen and a hydrocarbon gas selected from the group consisting of methane, ethane, propane, natural gas and mixtures thereof, in the presence of an inhibitor selected from the group consisting of the oxides, hydroxides, and chlorides of calcium; the oxides, hydroxides, and chlorides of strontium; and the oxides, hydroxides and chlorides of barium.

2. The method of claim 1 wherein the case hardening consists of carburizing said steel.

3. The method of claim 1 wherein said inert carrier gas contains hydrogen in addition to the nitrogen.

4. The method of claim 1 wherein said case hardening consists of carbonitriding said steel and wherein said gaseous atmosphere also contains ammonia.

5. The method of claim 1 wherein said gaseous atmosphere is passed over a bed of said inhibitor having a temperature of at least 200 F. prior to coming in contact with the steel to be case hardened.

6. The method of claim 1 wherein said inhibitor, in finely divided powdered form, is dusted on the surface of the steel to inn 

2. The method of claim 1 wherein the case hardening consists of carburizing said steel.
 3. The method of claim 1 wherein said inert carrier gas contains hydrogen in addition to the nitrogen.
 4. The method of claim 1 wherein said case hardening consists of carbonitriding said steel and wherein said gaseous atmosphere also contains ammonia.
 5. The method of claim 1 wherein said gaseous atmosphere is passed over a bed of said inhibitor having a temperature of at least 200* F. prior to coming in contact with the steel to be case hardened.
 6. The method of claim 1 wherein said inhibitor, in finely divided powdered form, is dusted on the surface of the steel to be case hardened prior to coming in contact with said gaseous atmosphere.
 7. The method of claim 1 wherein the inhibitor is a compound, metal or alloy of barium.
 8. The method of claim 1 wherein the inhibitor is barium hydroxide.
 9. The method of claim 1 wherein the hydrocarbon gas is natural gas.
 10. The method of claim 9 wherein said gaseous atmosphere comprises about 2 to 12 percent by volume hydrocarbon gas and the remaining 88 to 98 percent is the carrier gas. 